DE3856099T2 - Process for the production of low dielectric polyimides - Google Patents

Description

The present invention relates to a process for producing a film or coating from a low dielectric polyimide. Linear aromatic systems, particularly linear aromatic polyimides, were used to illustrate the teachings of the present invention. Some novel polyimides were prepared to verify the present invention.

US-A-4595548 discloses a process for producing a high temperature resistant, highly optically transparent to colorless aromatic polyimide film by a chemical reaction of an aromatic diamine such as 1,3-bis(aminophenoxybenzene) with an aromatic dianhydride such as 2,2-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride.

High performance polymer film and coating materials are increasingly used in electronic circuitry. As cited by Senturia (Proc. of ACS Polym. Matis. Sci. and Eng., Vol. 55, 385, 1986), polyimides are used in the microelectronics field for four main applications: (1) as manufacturing aids such as photoresists, planarization layers, and ion implantation masks; (2) as passivating overcoats and interlevel insulators; (3) as adhesives, and (4) as substrate components. Of utmost importance to the performance of a polymer used in electronic applications is its electrical behavior. In particular, to be useful as a passivating or protective overcoat, the material must be an excellent insulator.

The dielectric constant of commercially available polyimides currently used as state-of-the-art materials for passivators and interlevel dielectrics ranges from approximately 3.2 to 4.0 (depending on frequency and moisture content). The lower limit of 3.2 is obtained with a commercially available polyimide film Dupont Kapton H-Film only after complete drying. Unfortunately, as the film or coating absorbs moisture, the dielectric constant increases, complicating measurements and operation of electronic devices.

It is therefore a primary object of the present invention to provide a process for producing a low dielectric polyimide film and coating materials.

Another main object of the present invention is to provide a method for reducing the dielectric constant of aromatic condensed polyimides.

Another object of the present invention is to provide a process for producing films and coatings from aromatic condensed polyimides having a dielectric constant in the range of 2.4 to 3.2.

Another object of the present invention is to provide a process for producing films and coatings from low dielectric polyimides which are resistant to moisture.

Another object of the present invention is to provide novel polyimides which verify the aforementioned methods and which have a special Used in electronic applications as well as in industrial applications and in aerospace applications where high electrical insulation, moisture resistance, mechanical strength and thermal stability are required.

These objects are achieved by the process for producing a low dielectric high temperature resistant linear aromatic polyimide film and the aromatic polyimide as defined in the appended claims.

This procedure includes:

a) chemically reacting equimolar amounts of an aromatic diamine reactant and an aromatic dianhydride reactant in a solvent medium to form a high molecular weight polyamic acid solution;

b) minimizing the electronic interactions between polymer chains of the high molecular weight polyamic acid by means selected from the group consisting of:

i) incorporating separating or linking groups, selected from the group consisting of ether, sulfide and carbonyl groups into at least one of the aromatic diamine or aromatic dianhydride reactants prior to their use in the process,

ii) creating physical kinks or dissymmetry in the polymer chains of the high molecular weight polyamic acid by using a meta-linked aromatic diamine as the aromatic diamine reactant, and

iii) incorporating a bulky group selected from the group consisting of -CF3 and -SO2 into at least one of the aromatic diamine or aromatic dianhydride reactants prior to its use in the process to hinder interaction between the polymer chains;

c) incorporating one or more fluorine atoms into both the aromatic diamine and the aromatic dianhydride reactants in the form of -CF₃ or -C(CF₃)₂ groups or both prior to their use in the present process;

d) forming a film from the resulting solution of the high molecular weight polyamic acid or a solution of the corresponding polyimide in a solvent; and

e) thermally curing the film at a temperature up to about 300°C for a period of at least one hour to remove the solvent and to give a low dielectric high temperature resistant linear aromatic polyimide film.

The preparation of low dielectric polyimide films and coatings according to the present invention involves the conventional reaction of an aromatic diamine in a solvent with an aromatic dianhydride as follows: Room temperature amide solvent solid polyamic acid thermal curing optically transparent polyimide film

where R is a fluorinated group such as

and where Ar is a fluorinated group such as

In the above reaction, a polymer-grade aromatic diamine is dissolved in a dry amide-type solvent such as dimethylacetamide (DMAc). A polymer-grade aromatic dianhydride is added to the diamine solution at room temperature to form a polyamic acid. This resin is then sprayed onto a glass plate to form a film using a doctor blade with a specified blade pitch. The polyamic acid film is then thermally converted to the polyimide by heating at 250-300°C.

Another method of making a film or coating of a low dielectric polyimide of the present invention is by dissolving a polyimide powder in a solvent and using this solution to cast a film or spray coat it. The polyimide powder is prepared by chemically imidizing the polyamic acid in solution (the preparation of the polyamic acid is described above). The polyimide is then precipitated by pouring the imidized polymer dissolved in an amide solvent into a vigorously stirred nonsolvent such as water. The resulting polyimide powder can then be dried and stored indefinitely. When used, the polyimide powder is again dissolved in a solvent of choice and either cast into a film or spray coated onto a substrate.

Tables I, II, III, IV and V below illustrate the dielectric properties of various polyimides. In these tables, the polyimides which are outside the scope of the claims are marked with an "asterisk". These polyimides are examples which are useful for understanding the invention. These polyimides are prepared from dianhydride and diamine reactants which are not both fluorinated. The non-fluorinated groups R and Ar, which represent non-fluorinated dianhydride and diamine reactants are used,

R is

Ar is

The reduction of electronic interactions between polymer chains to produce low dielectric polyimides by adding separating or linking groups to the dianhydride is shown in Table I. The dielectric constants were measured at 10 GHz at room temperature and at a relative humidity of approximately 35%. As a basis for comparison, a commercial Kapton H film made from PMDA and 4,4'-ODA (chemical structure shown in Table 1) was also used, since it is widely used in the electronics industry as a film or coating material. Table 1: Dielectric properties of polymers containing oxydialine (ODA)

a the films were dried before examination.

b the measurements were carried out at room temperature and a relative humidity of approximately 25%.

c the value in brackets was obtained using commercially available Kapton H film.

* not part of the invention

The incorporation of a linking group or atom between two phenyl groups in the dianhydrides BTDA and ODPA reduces the acidity of these dianhydrides relative to PMDA, resulting in a reduction in the overall chain-chain electronic interaction. The dielectric constants of 3.15 for BTDA + 4,4'-ODA and 3.07 for ODPA + 4,4'-ODA are lower than those for PMDA + 4,4'-ODA (3.22) prepared in this laboratory or for commercial Kapton (3.25), which is reported to have a high degree of interchain electronic interactions. The addition of another phenoxy group to the dianhydride reduces the dielectric constant of HQDEA + 4,4'-ODA even further (3.02). The introduction of both -O- and -S- between phenyl rings decreases the dielectric constant even further, as in the case of BDSDA + 4,4'-ODA (2.97). Although in the present invention the above-mentioned -O- and -S- atoms and the -C=O group were used to separate the charge and reduce the acidity of the dianhydride, other groups could foreseeably be used.

The incorporation of physical "kinks" or dissymmetry into the polymer chain to reduce the electronic interactions between the chains and lower the dielectric constant is also in The dielectric constants are shown in Table 1. The many examples of polymers prepared with ODA in this table demonstrate that incorporation of meta-oriented (3,3'-) diamines into the molecular structure causes a reduction in the dielectric constant. Similar results are shown in Table II, where the last polymer in the table, 6FDA + 3-BDAF, prepared with a meta-oriented diamine, exhibits a dielectric constant of 2.40 compared to 2.50 for its para-oriented counterpart. Although ODA and BDAF were used in the specific examples to demonstrate the effect of meta-orientation on the dielectric constant of the polyimide, other meta-oriented diamines could predictably produce the same results when compared to their fully para-oriented (4,4'-) counterparts. Although meta isomerism was used in the present invention to create structural "kinks" or dissymmetry in the polymer chain and thus reduce the dielectric constant of the resulting polyimide, other forms of isomerism could predictably produce similar results. Other forms of isomerism in the aromatic diamine could include any combination of ortho, meta, or para isomerism so as to reduce symmetry and discourage any electronic interaction between the chains.

Table II: Dielectric properties of polymers made from BDAF diamines

where n=5-100

a 4-BDAF refers to both amine groups that are bound in the para position

b 3-BDAF refers to both amine groups that are bound in meta position

* not part of the invention

The electronic interactions between the polymer chains are also reduced by the incorporation of bulky groups along the main chain of the polyimide, which sterically hinder these interactions. The bulky groups used in the present invention are -CF₃ groups in the form of -C(CF₃)₂. It can be seen from Tables I, II and III that incorporation of such a group into the dianhydride and diamine produces polymers with relatively low dielectric constants. However, the bulky groups used in the present invention also contain fluorine atoms which satisfy the second condition previously stated for the preparation of a low dielectric polyimide. Table III lists the dielectric constants for a series of polymers prepared from 6FDA containing the bulky fluorinated -C(CF₃)₂ group. The dielectric constant is lowest when the -CF₃ groups are located in both the dianhydride and diamine portions of the polymer. Table III: Dielectric properties of polymers made from 6FDA dianhydrides

* not part of the invention

The incorporation of bulky groups such as -CF3 or -SO2 into a linear aromatic polyimide to reduce electronic interactions between the chains was previously reported by St. Clair and St. Clair (US Patent 4,603,061). This method was used to reduce charge transfer between chains and thus to produce optically transparent films. The most largely optically transparent or colorless film produced according to this patent was made from 6F (referred to here as 6FDA) and DDSO2. Although the use of the bulky -SO2 group was quite successful in reducing charge transfer between chains to the extent of producing a water-clear/colorless film, it was not equally successful in reducing the dielectric constant.

It is therefore not obvious to one of ordinary skill in the art that reducing the electronic interactions between the chains by incorporating bulky groups as taught by St. Clair and St. Clair in the above-identified patent should reduce the dielectric constant or make a polyimide more insulating based on the fact that this process produced aromatic polyimides with improved transparency. It is demonstrated in the present invention that for a For maximum insulating capacity, the aromatic polyimide must contain fluorine atoms in the polymer backbone. For example, as shown in Table IV, substitution of a bulky -SO2 group for -O- in Kapton's aromatic ODA diamine reduces chain-chain interaction, making the PMDA + DDSO2 polymers more transparent (pale orange). The dielectric constant remains the same as Kapton, however, incorporation of the 6FDA fluorine-containing dianhydride into the polymer to replace PMDA reduces the dielectric constant to 2.86. Substitution of 4-BDAF for ODA in the Kapton polymer produces a significant reduction in the dielectric constant to 2.63. When both monomers contain -CF3 groups, the dielectric constant is minimal (2.4 to 2.5).

The most successful method to reduce the dielectric constant of an aromatic polyimide is the combination of two conditions: (1) reducing chain-chain interactions and (2) incorporating fluorine into the polymer backbone. The best example of this success is that of 6FDA + 3-BDAF. This polymer has a minimum dielectric constant of 2.40 at 10 GHz. Both monomers contain fluorine atoms. The electronic interactions between the polymer chains should be minimal due to the -O- separating atoms in the BDAF diamine, the meta-isones in the 3-BDAF diamine, and the steric hindrance due to bulky -CF₃ units in both monomers. Table IV: Properties of a polyimide film

* not part of invention

The installation Incorporation of the fluorinated alkyl groups -CF₃ into the polyimides of this invention produces low dielectric materials which exhibit unusually excellent resistance to moisture, having properties similar to those of polytetrafluoroethylene. As previously mentioned, commercially available Kapton® polyimide film absorbs moisture, resulting in an increase in its dielectric constant. As shown in Table V, the dielectric constant of Kapton® at 1 MHz increases from 3.38 at a relative humidity of 30-35% to 3.85 at a relative humidity of 100% (an increase of approximately 14%). However, the dielectric constants of polymers of the present invention containing -CF₃ groups were significantly more stable when the films were exposed to increasing humidity. The dielectric constant of 6F + 4-BDAF changed by only 1% when exposed to 100% humidity. The dielectric constants at 100% relative humidity were measured for all films after immersing the films in distilled water for 24 hours.

Linear aromatic polyimides were used herein as exemplary systems to illustrate the teachings of the present invention. All of these polymers used as examples contain a five-membered imide group

and are therefore referred to as polyimides. However, the teachings of the present invention do not believe that the "imide" portion of this system contributes to the reduction of the dielectric constant of the polymer. Accordingly According to the teaching herein, to produce a low dielectric aromatic polymer, only conditions selected from the following must be met:

(1) Reducing electronic interactions between polymer chains by incorporating separating or linking groups such as ether, sulfide or carbonyl into the molecular structure of the polymer; incorporating physical linkages or dissymmetry into the polymer main chain; incorporating bulky groups such as -CF₃ or -SO₂ to hinder interactions between the chains; and

(2) Incorporation of fluorine atoms into the molecular structure of the polymer using -CF3 and/or -C(CF3)2 groups.

Therefore, by applying this method, the dielectric constant of any aromatic polymer could be predictably lowered. If all the above conditions are met, the dielectric constant of the aromatic polymer is minimized.

Although aromatic polyimide films and film coatings were used as physical samples to demonstrate the teachings of the present invention, other forms of the subject polymer could foreseeably be used. The dielectric constant of a polymer is inherently an inherent physical or intrinsic property. Therefore, powders, moldings, fibers, flakes, or other forms of a polymer could be used to demonstrate the teachings herein. Films were the preferred form for use in measuring dielectric constant because this form of polymer allows for complete solidification. It is additionally possible to perform dielectric measurements on polymers in film form. which are both highly accurate and reproducible. Table V: Dielectric properties of polyimide films measured at different relative humidities (RH)

* not part of the invention

SPECIFIC EXAMPLES Example I

To a dry vessel were added 1.0369 g of polymer grade (recrystallized) 2,2-bis(4(4-aminophenoxy)phenyl)hexafluoropropane (4-BDAF) (m.p. 1.62°C) and 10.9 g of dry dimethylacetamide (DMAC). After the diamine dissolved, 0.8885 g of polymer grade (recrystallized) 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) (m.p. 241°C) was added all at once to the stirred diamine/DMAC mixture. Stirring was continued until all of the dianhydride had dissolved and a solution with an inherent viscosity of 1.0 dL/g at 35°C was obtained. The resulting polyamic acid solution (15 weight percent solids) was cooled until use for film casting.

A film of the 6FDA + 4-BDAF polyamic acid was prepared by casting the resin onto a soda-lime glass plate in a dust-free chamber at a relative humidity of 10%. The solution was spread using an aluminum knife with the gap adjusted to ensure a final film thickness of 1.0 mil. The polyamic acid film was thermally converted to the corresponding polyimide by heating in a forced air oven for 1 hour at 100°C, 200°C, and 300°C, respectively. After cooling to room temperature by immersion in warm water, the resulting film was removed from the glass plate. The dielectric constant of this film measured under ambient conditions was 2.77 at 1 MHz and 2.50 at 10 GHz. The film was found to be essentially resistant to moisture. It was found that the dielectric constant of 6FDA + 4-BDAF did not vary by more than 1% after the film was immersed in distilled water for 24 hours.

Example II

Using the same procedure and conditions as in Example 1 of the present invention, equimolar amounts of 2,2-bis[4(3-aminophenoxy)phenyl]hexafluoropropane (3-BDAF) (mp 133°C) and 6FDA were dissolved in DMAC to form a polyamic acid resin having an inherent viscosity of 0.53 dl/g. The resulting 6FDA + 3-BDAF polyimide film had a dielectric constant of 2.40 at 10 GHz. Like that of the 6FDA + 4-BDAF film of Example 1, the dielectric constant of 6FDA + 3-BDAF changed very little when the film was exposed to 100% relative humidity.

Example III

Following the same procedure and conditions described in Example I of the present invention, equimolar amounts of 2,2-bis(4-aminophenyl)hexafluoropropane (4,4'-6F) (mp 194°C) and 6FDA were dissolved in DMAC to form a polyamic acid with an inherent viscosity of 1.0 dl/g. The resulting polyimide had a dielectric constant of 2.39 at 10 GHz.

Example IV

By the same procedure and conditions described in Example I of the present invention, equimolar amounts of 2,2-bis(3-aminophenyl)hexafluoropropane (3,3'-6F) (mp 79°C) and 6FDA were dissolved in DMAC to form a polyamic acid having an inherent viscosity of 0.60 dl/g. The resulting polyimide film had a dielectric constant of 2.50 at 10 GHz.

Example V

A viscous polyamic acid resin was prepared from 6F + 4-BDAF using the same procedure and conditions as described in Example 1 of the present invention; and an imide powder was prepared from this preparation by chemical imidization of the polyamic acid using pyridine/acetic anhydride, which is well known in the art. The powder was precipitated by pouring the imidized polymer/DMAC solution into vigorously stirred distilled water. The resulting white powder was dried at 60°C for 4 hours and at 200°C for 2 hours. According to the A film was prepared by casting a solution of the 6F + 4-BDAF powder dissolved in chloroform following the procedure described in Example I. The resulting film was of good quality and had a dielectric constant of 2.55 at 10 GHz.

Claims (12)

1. A method of producing a low dielectric high temperature resistant linear aromatic polyimide film, the method comprising: a) chemically reacting equimolar amounts of an aromatic diamine reactant and an aromatic dianhydride reactant in a solvent medium to form a high molecular weight polyamic acid solution; b) minimizing the electronic interactions between polymer chains of the high molecular weight polyamic acid by means selected from the group consisting of: incorporating separating or linking groups, selected from the group consisting of ether, sulfide and carbonyl groups into at least one of the aromatic diamine or aromatic dianhydride reactants prior to their use in the process, ii) creating physical kinks or dissymmetry in the polymer chains of the high molecular weight polyamic acid by using a meta-linked aromatic diamine as aromatic diamine reactant, and iii) incorporating a bulky group selected from the group consisting of -CF₃ and -SO₂ into at least one of the aromatic diamine or aromatic Dianhydride reactant prior to its use in the process to hinder the interaction between the polymer chains; C) incorporating one or more fluorine atoms into both the aromatic diamine and the aromatic dianhydride reactants in the form of -CF₃ or -C(CF₃)₂ groups or both prior to their use in the present process; d) forming a film from the resulting solution of the high molecular weight polyamic acid or a solution of the corresponding polyimide in a solvent; and e) thermally curing the film at a temperature up to about 300°C for a period of at least one hour to remove the solvent and to give a low dielectric high temperature resistant linear aromatic polyimide film. 2) A method according to claim 1, wherein the solution of the high molecular weight polyamic acid is applied to a surface and the applied film layer is thermally cured in the temperature range of 250°C to 300°C for a period of at least one hour. 3. The process of claim 1, wherein the aromatic diamine is selected from the group consisting of: 2,2-bis(4(3-aminophenoxy)phenyl)hexafluoropropane, 2,2-bis(4(4-aminophenoxy)phenyl)hexafluoropropane, 2,2-bis(4-aminophenyl)hexafluoropropane, and 2,2-Bis(3-aminophenyl)hexafluoropropane. 4. The process of claim 1, wherein the aromatic dianhydride is 2,2-bis(3,4-dicarboxylphenyl)hexafluoropropane dianhydride. 5. The method of claim 11 wherein the solvent medium is selected from the group consisting of: N,N-dimethylacetamide, N,N-dimethylformamide, N-methyl-2-pyrrolidone, and Dimethyl sulfoxide 6. The method of claim 1, wherein: the resulting solution of high molecular weight polyamic acid is chemically imidized to produce a precipitate of the corresponding polyimide; the corresponding polyimide is then dissolved in a solvent and a film layer prepared from the solution of the polyimide in the solvent is applied to a surface; and the film layer is then heated to a temperature range of 20ºC - 300ºC to remove the solvent and produce a high temperature resistant low dielectric polyimide film. 7. The process of claim 6, wherein the high molecular weight polyamic acid solution is chemically imidized by a process which comprises adding a solution of acetic anhydride and pyridine in an organic solvent to the high molecular weight polyamic acid solution. 8. The method of claim 6, wherein the solvent used to dissolve the polyimide is selected from the group consisting of: N,N-dimethylacetamide, N,N-dimethylformamide, N-methyl-2-pyrrolidone, Dimethyl sulfoxide, chlorinated solvents, Tetrahydrofuran, m-Cresol, Methyl ethyl ketone, Methyl isobutyl ketone, and Bis(2-methoxyethyl) ether. 9. A process according to claim 6, wherein the polyimide precipitate is mixed with freshly distilled water, washed and thoroughly dried at 60°C and at 200°C for 2 hours before being dissolved in a solvent. 10. A high temperature resistant insulating aromatic polyimide having a dielectric constant in the range of 2.4 to 3.2, which is produced by the process according to claim 1. 11. A polyimide film according to claim 10, which does not lose its insulating properties when exposed to large humidity changes. 12. A low dielectric polymer comprising a linear aromatic polyimide which is a reaction product of 2,2-bis (3, 4-dicarboxyphenyl) hexafluoropropane dianhydride with the aromatic diamine 2,2-bis(4(3-aminophenoxy)phenyl)hexafluoropropane is formed.